1. Field of the Invention
The present invention generally relates to the field of maskless lithography (MLL) and optical maskless lithography (OML).
2. Background Art
OML is an extension of conventional (i.e. mask-based) photolithography. In OML, however, instead of using a photomask, tens of millions of micro-mirror pixels on a micro-electro-mechanical systems (MEMS) device are dynamically actuated in real-time to generate the desired pattern. Due to a fixed grid imposed by the pixels and use of short-pulse duration excimer lasers at deep ultra-violet (DUV) wavelengths, spatial modulation of gray scales is required. These classes of MEMS devices are therefore known as spatial light modulators (SLMs).
SLMs may utilize one of several geometrical actuation types (e.g. tilt, piston, etc.) for creating images. By using the same wavelengths and resists as conventional mask-based photolithography scanners, OML is directly compatible with existing line facilities and can be integrated into existing fabrication facilities with the same track and etch equipment. OML provides more design turns in less time to facilitate optimization of chip yields and speed.
One important conceptual difference between OML and traditional mask-based lithography stems from the differences between the SLMs used in OML, and the traditional masks. The SLMs consist of many pixels, with each pixel having an ability to vary its optical properties in a controllable manner. Typically, each pixel is sub-resolution in size and can assume one of many possible states. Some SLM designs utilize the physical principles of light modulation that are not necessarily equivalent to the ones utilized in a design of traditional masks. For instance, tilting mirror pixels operate by varying the degree of light deflection, which is not necessarily equivalent to a fragment of a traditional mask.
In addition to defining the necessary specifications of the SLM itself, one of the main challenges in OML is to understand how best to use the SLM pixels to reproduce the desired pattern. Rasterization, which presents significant challenges in this area, is one technique used to configure SLM pixels to reproduce patterns. More specifically, given the description of a mask pattern or the desired properties of an image, it is not an easy task to determine the states of the SLM pixels that result in that mask pattern, or that most closely approximate the desired image.
Several approaches to OML rasterization have been described in the literature. One such approach is image optimization. Image optimization attempts to solve the optimization problem by performing iterations of the pixel states to optimally print a desired pattern while accounting for feature proximity effects and optical interference between the pixels. On a conceptual level, this approach follows many techniques utilized for optical proximity effect correction (OPC) features designed for traditional masks.
Because the objective function of the optimization can be formulated in terms of the properties of an aerial image (or possibly even in terms of the image in resist), the image optimization approach allows relatively precise control of the desired properties of the pattern to be printed. In principle, to achieve optimization, one can place an edge at a certain location, with the normalized intensity log slope (NILS) along this edge satisfying a specific threshold.
The major challenge associated with image optimization, however, is its comparatively high computational cost—each iteration typically involving re-evaluation of the aerial image and its derivatives with respect to the pixel states.
A second conventional approach known as off grid filter (OGF) works by selecting the states of the SLM pixels to approximate the diffraction field of the ideal mask being rasterized. In the OGF approach, this approximation is performed locally. That is, several neighboring pixels are utilized to approximate the diffraction field generated by the fragment of the mask covered by one or more of these pixels. Local approximation allows pre-computation of the states of these neighboring pixels (the grid filter coefficients corresponding to the selected filter stencil). An advantage of OGF is that it is comparatively fast, and can possibly be executed in real-time by applying a pre-computed filter to the given description of the pattern. However, some of the tradeoffs suffered to realize the advantages of OGF are as follows:    (i) Because the filter stencil necessarily involves a limited number of neighboring pixels, the match of the pupil field by the filtered pixel states will always be approximate, although the error of this approximation can be made relatively small for pixel dimensions that are small compared to the optical resolution length;    (ii) In many variations of the OGF approach, the filtered states of the pixels generally do not satisfy the constraints imposed on them by the SLM pixel design. One option is to introduce a scaling factor reducing the image intensity as much as necessary to satisfy the constraints. This option, however, introduces a light loss, which is generally not desirable.    (iii) In many variations of the OGF approach, the underlying requirement is that the modulated pixel should be well approximated by a graytone square. The graytone of such an approximating square should be real-valued and constant across the square. The first of these conditions is not valid for piston mirror pixels and the second condition is only approximated for a tilt mirror pixel.
One problem associated with conventional approaches such as OGF, is the issue of light loss. That is, given the pattern on an ideal mask to rasterizing, the conventional approaches provide a rasterization solution that reproduces a variation of the aerial image of the mask with this pattern. However, for many important patterns, the image that results from such rasterization is a scaled or dimmed image of the original pattern. For many patterns common to lithography (e.g., lines/spaces and contact holes), an image produced under the effects of light loss could contain as little as 1/7 of the dose of the original image resulting from the pattern on the original mask.
A solution to the issue of light loss is based on certain assumptions about the modulation capabilities of the SLM pixels. Namely, this solution requires that the field generated by the modulated pixel across the projection optics (PO) pupil and in the image plane is well approximated by the field generated by gray toning squares of an SLM with a real-valued graytone. These assumptions hold for many modulation principles (tilting mirrors, liquid crystal display (LCD) cells etc.) as long as the pixels are sufficiently under-resolved. However, these assumptions might not immediately be valid for pistoning mirror pixels (complex-valued graytones with unit amplitudes). They are also largely invalid in the case of other modulation types (e.g. tilting mirrors) when the dimensions of the sub-resolution pixels approach the resolution limit from lower values.
What is needed, therefore, is a technique to provide accurate rasterization with minimal light loss for SLM pixels that can be applied using various modulation principles.